If your client app written in Delphi, there is bad usage of Variant type in your code (or external components).

Absent one of checks when converting Variant to Currency, e.g.:

procedure TForm1.Button1Click(Sender: TObject);
var
   C: Currency;
   V: Variant;
begin
   V := 10;
   C := V; // Works
   Edit1.Text := FloatToStr(C);

   V := Null;
   C := V; // Error
   Edit1.Text := FloatToStr(C);

   V := Null;
   // Correct usage
   if VarIsNull(V) then C := 0 else C := V;
   Edit1.Text := FloatToStr(C);  
end;